CRUSHING AND GRINDING 193
Angle of nip in its broader sense may be said to be the angle formed between the planes of crushing surfaces acting upon the particle to be crushed. For example, in the case of gravity stamps, the crushing surfaces are parallel and angle of nip is zero and need not be considered. On the other hand, in breakers and rolls the crushing surfaces are not parallel and the ore particles to be crushed are held between two inclined surfaces approaching one another and exerting pressure upon the ore particle. To those engaged in rock or ore crushing, there are two conditions of angle of nip which are of special interest. The first may be termed "the critical angle of nip" and the second may be called the "safe" or "practical" angle of nip. The critical angle of nip may be defined as that angle formed by the crushing surfaces, at which the ore particle is just on the dividing line between being held between the crushing surfaces and being shot out from between them by pressure. Or, in more scientific language, it is that angle where the components of the forces, including friction, acting to hold the particle in between the crushing surfaces, are just balanced by the components of the forces tending to snap the particle outward. "The safe or proper angle of nip" is a somewhat less angle than the "critical angle" and represents the angle between the crushing surfaces at which there is practically no possibility that. the ore particle can be snapped out. This angle is a variable depending upon various factors but, for general average conditions, in the case of crushing rolls, Richards ("Ore Dressing," Vol. Ill, p. 1226) has estimated that 32 deg. represents the best figure to be taken. Since rolls have curved surfaces, the only way that angle of nip can be figured in their case is to take that angle formed by the intersection of the two tangents drawn between the two points of contact of the ore particles against the •rolls.1
In the case of breakers, both of the jaw and gyratory types, the angle of nip must also be considered and it is to be presumed that the general angle of 32 deg. applies equally well. The factors which alter the angle, that is, the safe working angle, are the character of the ore, the character of the crushing surfaces, the speed of approach of the crushing surfaces and perhaps, in some cases, also the way that the rock is fed. The character of the ore affects it because a smooth, hard particle or a soft, greasy particle will have a lower coefficient of friction than an average gritty particle of rock and will, therefore, require a less angle of nip than 32 deg. as the safe angle. Similarly, crushing surfaces which have become worn smooth and are of a hard, glassy nature will have a lower coefficient of friction than the average crushing surfaces and will require reduction of angle of nip. The effect of speed is perhaps less than that of the two preceding factors, although it will be found in general that where crushing surfaces are moving toward one another very rapidly, the coefficient of friction is, in effect, reduced by this high speed and thus a lowering of the angle nip brought about. This is very clearly brought out at times by crushing rolls where ore particles will not be nipped while the rolls are running, but will frequently become nipped after the rolls have been disconnected from power and just before they come to a stop. The effect of manner of feeding may have also a minor effect. If rock is fed dry, the coefficient of friction is higher than it is if the material is fed with a little moisture in it. On the other hand, if the rock is fed with a stream of water, then this stream of water may wash away lubricating clay, etc., and have a tendency to increase the coefficient of friction. Speed of feeding likewise may have minor effect in that an ore particle fed upon swiftly moving crushing surfaces is not immediately accelerated to the speed of that surface and hence may fail to be nipped. This is especially true in the case of crushing rolls and means that the high-speed rolls usually require the use of an angle of nip somewhat less than 32 deg., in order to be safe.
Rock-breaking Machinery.—In the field of primary rock breaking there are
See RICHARDS, "Ore Dressing," Vol. Ill, p. 1226, and Vol. IV, p. 89. 13